The basics of fiber-optics--part 1

Learning fiber optics is much easier than learning the laws of electricity. You've probably heard the terms "light tube" and "conduit for light" in reference to an optical fiber. These terms might lead you to think there's some type of hole in the middle of an optical fiber. This is not the case. Looking at a typical optical fiber cross section helps explain its operation.There are three concentric

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Learning fiber optics is much easier than learning the laws of electricity. You've probably heard the terms "light tube" and "conduit for light" in reference to an optical fiber. These terms might lead you to think there's some type of hole in the middle of an optical fiber. This is not the case. Looking at a typical optical fiber cross section helps explain its operation.

There are three concentric layers to an optical fiber. Light pulses only through the glass core of the fiber. The cladding (which is a different type of glass) serves as a barrier to keep the light within the core, functioning much like a mirrored surface. The buffer layer (sometimes called the coating) has nothing to do with light transmission and is used only for mechanical strength and protection.

As you can see, light does flow down the center of a fiber like water through a pipe. We could even say that the fiber is a "virtual" tube. Light stays in the center of the fiber, not because there is a physical opening there, but because the cladding glass reflects any escaping light back to the core

These "light tubes" are very thin strands of ultra-pure glass. The dimensions of typical fiber components are:

core: 8 microns to 62.5 microns (a micron is one millionth of a meter)

cladding: 125 microns

coating: 250 microns

The core is one density of glass, the cladding is a second grade of glass, and the coating is a plastic.

Cabling

To suitably protect our glass fibers, we package them in cabling. It's a misconception that fiber cables are fragile. Many people believe if you drop a fiber cable on a hard surface, it will shatter-after all, it's made of glass! But, this is a myth.

The fiber itself (not the cable, but only the thin fiber) is fairly fragile, although it's surprisingly flexible and will not break easily like sheet glass. (Actually, it's several times as strong as steel, but because it is so thin, it can be broken fairly easily.)

Fiber cables, however, are not fragile at all. In fact they're often more durable than copper communication cables. Optical cables encase the glass fibers in several layers of protection.

The first protective layer is the coating we mentioned earlier. The next layer is a buffer layer, which is typically extruded over the coating to further increase the strength of the single fibers. This buffer can be of either a loose tube or tight tube design. Most datacom cables are made using either one of these two constructions. A third type, the ribbon cable, is frequently used in the telecommunications work and may be used for datacom applications in the future. It uses a modified type of tight buffering.

After the buffer layer, the cable contains a strength member. Most commonly, it's a Kevlar fabric, the material used in bulletproof vests. The strength member protects the fiber and also carries the tensions of pulling the cable. (You should never pull fiber cable by the fibers themselves.)

After the strength member comes the outer jacket of the cable, which is typically some type of polyethylene or PVC. In many cases, however, there will be additional stiffening members, which also increase the cable's strength and durability.

Important field-installed components As with copper cabling, you have field-installed components used to make a working datacom system.

Connectors. Fiber connectors are used to make nonpermanent connections at fiber ends. Because they have such as small diameter, you must hold optical fibers rigidly in place and accurately align them to mate with other fibers, light sources, or light detectors.

Advances in design and technology make connector installation easy today. But, it wasn't always this way.

As the fiber-optic field began to develop, one of the biggest mechanical problems was how to permanently fix the fibers at their ends. The first fiber connectors were difficult to install. They used a variety of glues, ovens, and long, difficult polishing methods. Since then, things have drastically changed.

Although terminating a fiber is not yet as easy as installing a coaxial cable connector, it's far, far easier than before. In fact, you can terminate a fiber in about half the previous required time, and the process continues to get easier as time goes on. Within a few years it should be quite simple.

Splices. You use splices to permanently join the ends of fibers. There are two primary ways this is done: by fusion (the melting of pieces of glass together) and mechanical means. When installing a splice, you must address two critical factors:

The fiber joint must be able to pass light without loss and the joint must be mechanically secure so that it won't be easily broken.

Testing When installing a fiber system (the whole system of optical fibers is often called a cable plant), you must test it to verify its performance. Basically, you're making sure light will pass through the system properly. There are three types of optical testing: Continuity, power, and OTDR testing.

Continuity testing. This is a simple visible light test. Its purpose is to make sure the fibers in your cables are continuous (unbroken). You do this with a modified type of flashlight device and the naked eye. It takes only a few minutes to perform.

Power testing. This accurately measures the quality of optical fiber links. As shown in Fig. 3, a calibrated light source puts infrared light into one end of the fiber and a calibrated meter measures the light arriving at the other end of the fiber. You measure the loss of light in the fiber in decibels.

OTDR testing. This testing uses a piece of equipment properly called an Optical Time Domain Reflectometer, or OTDR. This device uses light backscattering to analyze fibers. Basically, the OTDR takes a snapshot of the fiber's optical characteristics by sending a high powered pulse into the fiber and measuring the light scattered back toward the instrument. You can use an OTDR to locate fiber breaks, splices, and connectors as well as to measure loss. The OTDR method may not give the same value for loss as a source and power meter due to the different methods of measuring loss. However, the OTDR gives a graphic display of the status of the fiber under test. Another advantage is it requires access to only one end of the fiber.

As useful as the OTDR is, it's not necessary in the majority of situations. Also, it's quite expensive. Even when they are necessary, many installers prefer to rent rather than purchase them.

Technical concepts

In addition to the things we've covered so far, you must understand several technical concepts.

Attenuation. This performance characteristic is the measure of weakening of an optical signal as it passes through a fiber. In other words, it's a measure of signal loss. Attenuation in an optical fiber is a result of two factors: absorption and scattering.

Absorption is just that: the absorption of light and its conversion to heat by molecules in the glass. Primary absorbers include residual deposits of chemicals used in the manufacturing process to modify the characteristics of the glass. This absorption occurs at definite wavelengths. (Remember, the wavelength of light signifies its color and its place in the electromagnetic spectrum.) It's determined by the elements in the glass and is most pronounced at the wavelengths around 1000 nm, 1400 nm, and above 1600 nm.

Scattering is the largest cause of attenuation. It occurs when light collides with individual atoms in the glass and is knocked off its original course. Fiber optic systems transmit in the "windows" created between the absorption bands at 850 nm, 1300 nm, and 1550 nm wavelengths, for which lasers and detectors can be easily made.

Networks. To communicate between several pieces of equipment (for example, between 20 different computers in an office), you have to connect them together. To do this, you must:

Develop a logical method of connecting them. (Should they all be tied to a central point or connected in a loop?)

Provide a definite protocol for communicating. (If the machines don't "talk" to each other in some type of order, the whole system will collapse in a jumble of signals the machines can neither separate nor interpret.

There are many types of networks, each with their own strengths and weaknesses. In fact, you've probably heard of them: Ethernet, 10base T, FDDI, ATM, and Token Ring. These are simply different methods of connecting computers together.

Bandwidth. This is the range of signal frequencies or bit rate at which a fiber system can operate. Basically, it's a measure of the amount of signal able to be put through a fiber. Higher bandwidth means more data per second; lower bandwidth means less signal. Dispersion. There are two potentially confusing terms you'll come across in your readings: Chromatic dispersion and modal dispersion. In both of these terms, the word "dispersion" refers to the spreading of light pulses until they overlap one another. This distorts and causes the loss of the data signal.

Chromatic refers to color. Modal refers to the light's path. Therefore, chromatic dispersion is signal distortion due to color, and modal dispersion is signal distortion due to path. Dispersion is not a loss of light; it's a distortion of the signal. Thus, dispersion and attenuation are two very different and unrelated problems. Attenuation is a loss of light; dispersion is a distortion of the light signals.

Why use optical fiber, anyway? We use optical fiber to transmit all types of data and communications signals over all distances, short or long. Because the amount of data and communications signals used for business and personal use has grown exponentially in recent years, sending high amounts of signal has become necessary for modern life. In 1987, for example, an average home used about 3000 Hz worth of bandwidth (basically one telephone line). Now, it's not uncommon for the same home to use two 56 KHz modems transferring data; 14,400 baud for fax transmissions, several hundred MHz of cable television, and 3000 Hz of voice, all at the same time. This situation is often more dramatic for businesses.

As you can see, our need for more signal transmission capacity is urgent and increasing. The communications networks built of copper wire simply cannot keep up. Yes, copper is reliable and served us well for decades, but it cannot operate well at high signal transfer rates. For data transfer rates of above 50 million bits per second (Mb/s), you need special systems copper wiring. Above 150 Mb/s, even the best copper wiring is questionable.

Optical fiber, on the other hand, can handle transmission rates many times that high. At the time of this writing, systems operating at 40 billion bits per second (gigabits per second, or Gb/s) over a single fiber are common, and we are not really sure how high our bit rates will be able to go. We are certainly nowhere near the limit yet.

Another advantage to optical fiber is its electrical immunity. Because optical fiber cable is nonmetallic, it cannot emit or pick up electromagnetic interference (EMI) or radio frequency interference (RFI), each of which is a problem with metallic conductors.

Furthermore, cross talk between FO cables does not exist. Additionally, FO cabling has no grounding or shorting problems. This is also an important feature when installing communications wiring in hazardous environments. Finally, optical fiber cabling causes no sparking or excessive heat-even when broken.

The security of optical fiber is also far superior to that of copper wire. Electronic bugging depends on electromagnetic monitoring. Because optical fibers carry light rather than electricity, they are immune to bugging. In order to plant a bug on an optical fiber cable, you have to physically tap the cables. However, this is easily detectable because the signal diminishes and error rates increase.

Currently, the industry uses optical fiber for all long-distance telephone traffic and increasingly for local telephone circuits. Other communications systems, from cable television to computer networks, are changing over to optical fiber right now. Eventually, almost every communication signal will be sent over optical fiber.

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